boballab, the article “The Truth About Dogs” was interesting, thank you. The link goes to the last page of five, for those others that read it.
I’m not sure though if you think that I’m asserting that inbreeding doesn’t happen, I’m not. I picked up on the copy machine metaphor because that is not how sexual reproduction works.
Asexual reproduction, such as that performed by an amoeba, works by cells copying themselves, and though small mutations occur they are not introduced into the rest of the population. As to degradation, well simple creatures like these have been extremely stable for millions of years.
Cell division in larger organisms is not dissimilar to asexual reproduction and mutations happen here too. Sometimes they have no appreciable difference, sometimes the cells are better or left inert and others lead to abnormal masses of tissue or even cancers.
Sexual reproduction doesn’t copy it combines compatible cells. The advantages of sexual reproduction is that mutations can be spread throughout a population as haploid cells are contributed from two sources. This allows swifter adaptation and change to occur. It can also stabilise a mutated line. There are also a few error preventatives in the mating process, cell division and gestation itself.
From the article page 4:
http://www.theatlantic.com/doc/199907/dog-genetics/4
Today, when an unscientific embrace of "biodiversity" is almost as common as the unscientific embrace of "racial purity" was a century ago, inbreeding is often portrayed as an unmitigated evil. But that is almost as much an oversimplification as the uncritical embrace of purity for purity's sake was. Inbreeding has in fact been a vital technique in the development of virtually every strain of plant and animal useful to agriculture, and it is the only way to rapidly develop a line that will consistently produce certain desirable characteristics. This is at heart a consequence of the biological fact that chromosomes come in pairs; one is inherited from each parent. Closely related individuals—brothers and sisters, parents and offspring—are more likely to carry the same genes. So a mating between two closely related individuals increases the likelihood that the offspring will wind up with the same gene for a given trait on both chromosomes—a state called homozygosity. An organism that is heterozygous for a given trait—that is, has different versions of the gene on each chromosome—may look the same as one that is homozygous, but it will not pass that trait to its offspring as consistently. In the classic human example, both a homozygous individual and a heterozygous one can have brown eyes, though the latter has one gene for brown eyes and one for blue eyes. Brown is "dominant" in this case. But the "recessive" (blue) genes carried by two heterozygous individuals may combine in reproduction to produce offspring who are homozygous for the recessive trait and who will thus be different in appearance—a person with two blue genes has blue eyes. With a homozygous mating, though, what you see is what you get. No matter which of each parent's pair of chromosomes gets passed on to the offspring, the result is the same. In other words, homozygotes breed "true to type" for the traits they have been selected for.
But since closely related individuals have a lot of other genes in common too, inbreeding also increases the chances that any genes for undesirable recessive traits carried at other sites on the genome will combine to produce trouble. Inbred faults in domestic animals tend to be recessive because genetic diseases caused by dominant traits are quickly weeded out in a breeding program: eliminate from the breeding population all the animals that manifest such a disease, and you eliminate the genes for that disease from the entire breeding population. (It takes but a single dominant gene to cause a dominant disease, so there are no "silent" carriers of such genes.) But genetic diseases that show up only in an animal homozygous for a recessive trait can be carried silently for generations. Only when two carriers happen to mate will the disease appear.
Inbreeding itself if not bad. It’s just when there is a disadvantageous mutation it’s harder to get rid of and can lead to immediately apparent results.
Even here though we are already oversimplifying. Genetic traits come from small variations from many different parts, not just simple “pairing”. By reducing a complex issue into condensed statements we introduce inconsistencies.
Yes, inbreeding can exacerbate genetic issues. However, new species can start with a single pairing.
Over time mutations occur to the genetic material. These changes may be good, bad or indifferent.
Small populations are systemic of populations approaching extinction. Small populations with a genetic advantage over others in the same species can come to be the dominant subspecies over time.
Life is full of paradoxes. Just because one thing is “true” it doesn’t mean its opposite isn’t also true. Reducing complex problems into simple concepts is much of the basis of language, and engineering, to great advantage in improving our understanding of the world.
Take an glider, reducing air resistance improves performance, yet the wing exploits differences in air flow to generate lift. Then in the real world foils (pun intended) have to be introduced to combat flows and eddies of air over the aerodynamic surfaces. There are three levels contrary engineering efforts going on. Genetics is vastly more complex.